Four-stroke and two-stroke combustion engines are widely used and each of them has features and also difficulties. The feature of the four-stroke engines is the good efficiency because the cylinder is cleaned out by force after each working stroke. Since at four strokes only one working stroke appears these engines have the disadvantage that they are of relatively small power per unit of weight.
The two-stroke engines have big power per unit of weight, because they have a working stroke at every two strokes. However, their efficiencies are bad, because the cylinders are not cleaned by force. Therefore, full filling with clean gas-air mixture is not assured.
A difficulty of both types of common engines is, that the cross-sectional ares for intake and exhaust are limited because there is not enough space for a maximum of inlet and exhaust area.